REDUCTION IN POSTSYNAPTIC INHIBITION DURING MAINTAINED ELECTRICAL-STIMULATION OF DIFFERENT NERVES IN THE CAT HINDLIMB

1. Steady-state postsynaptic potentials (PSPs) were generated by prolo nged(similar to 1 s) high-frequency( 100-200 Hz) electrical stimulatio n of nerves in the cat hindlimb. The characteristics of these steady-s tate PSPs were compared for two polysynaptic afferent pathways (ipsila teral cutaneous sural vs. contralateral peroneal nerves), two animal p reparations (decerebrate vs. chloralose), and two motoneuron pools(med ial gastrocnemius vs. lateral gastrocnemius-soleus). 2. PSPs from both nerves usually (36 of 51 cases) contained a mixture of depolarizing a nd hyperpolarizing components. In all 36 cases where the PSP contained a hyperpolarizing component, a consistent qualitative pattern emerged during prolonged stimulation: the hyperpolarization reached a peak si milar to 20 ms after stimulation onset and then decayed with a biphasi c time course that consisted of an initial rapid phase (20-40 ms) and a later slower phase (200-400 ms) before the steady-state value was re ached. This pattern occurred regardless of the differences in polysyna ptic afferent pathways, animal preparations, and motoneuron pools. 3. The consistency of this overall pattern was remarkable, given the exis tence of several quantitative differences among the PSPs. These differ ences include the following: hyperpolarizing components were least com mon in the sural and peroneal PSPs in the decerebrate preparation. And only these sural and peroneal PSPs tended to have prolonged afterpote ntials after stimulus cessation. The steady-state sural PSPs in the de cerebrate preparation tended to generate the largest PSPs and, moreove r, these PSPs did not follow the overall trend of having a statistical ly significant relation between the amplitude of the initial hyperpola rization and the amount of its decay. Finally, transient sural PSPs in lateral gastrocnemius-soleus motoneurons in the decerebrate preparati on tended to have the largest hyperpolarizations. 4. To determine whet her the decay of the hyperpolarization and the subsequent dominance of depolarization was due to a decreased inhibition or an increased exci tation, injected current pulses were utilized to measure the changes i n the cell's input resistance during the course of the synaptic input. A strong decrease in input resistance accompanied the initial period of maximal hyperpolarization (50% with respect to the resting input re sistance). Input resistance then returned toward resting values as hyp erpolarization faded and depolarization became dominant. However, ther e remained a persistent decrease in input resistance during the final phase of the PSP that amounted to <10% of the initial decrease. These findings indicated that much of the reduction in hyperpolarization ref lected a progressive decrease in synaptic efficacy for the inhibition. 5. A portion of the decline in hyperpolarization, however, did appear to be consequent to a slow increase in the synaptic efficacy for the excitation, because the initial decrease in hyperpolarization followed a more rapid time course than did the changes in input resistance. 6. The reduction in inhibition during maintained input suggests that the role of inhibition from some input sources may be limited to transien t conditions, such as the changes in cutaneous input during locomotion . The potential role of the reduction in sural inhibition in limiting the duration of recruitment order reversals is considered.